some creationist claim the 3 digits of birds aren't evidence for evolution because they use from their like hoatzin. Below picture
Is this true and these digits is useful not vestigial?

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170px-Hoatzin_chick.jpg, 10.41 kb, 170 x 222

A vestigial feature is not necessarily something that is not used.  Typically, an organ will be classified as vestigial if its original use or size has been dramatically reduced. 
Hoatzin chicks have claws that extend past the feathered portion of their wings.  However, as their wings grow, they cover the claws.  Adult hoatzins have fairly typical looking bird wings.  Think about the wing/body ratio of a baby bird compared to a adult bird, the wings get a lot bigger compared to body size as the bird gets older. 

I link to a figure from a Nature article that shows drawings of fossil hands as well as a hypothesis as to the evolution of the development of reduced digits in birds and therapod dinosaurs.

http://www.nature.com/ncomms/journal/v2 … 37_F1.html